The counting of layers such as tree rings, glacial varves, and annual ice layers, is a reliable way of establishing a chronology.
As far as the age of Earth is concerned, these methods provide a minimum age for Earth. Scientists do use radiometric dating to calibrate these methods from time to time—C-14 dating of some individual tree rings is an example—but layer counting firmly establishes an annual or seasonal count from the present into the past. With regard to the minimum earth age established by layer-counting, radiometric dating does not come into play.
How many layers have been counted?
Trees have been found having rings showing an age between 4000 and 5000 years, and some of these trees died a long time ago. By matching chemical and environmental qualities among trees at different ages, it is possible to construct a continuous environmental history to between 11,000 and 13,000 years ago.
Glacial varves (the layered sediment at lake bottoms near continuously melting glaciers) have been found that clearly show 60,000 layers at Lake Suigetsu in Japan. Below this level, the layers are less distinguishable, but the age could be 150,000 to 200,000 years if you assume a uniform rate of sedimentation.
In 2007, scientists working in Antarctica cut out deep cores of ice from huge ice formations to learn about the climate of the past, providing approximately 740,000 years of history. But I was surprised to read that layer-counting was not done there in central Antarctica, because annual cycles are barely distinguishable. Instead, the history is dated by estimating the snow accumulation rate (together with information about static compression) and by matching various markers to other studies done previously. The researchers were pleased to note that up to 100,000 years, the markers provide excellent pattern-matching with two other Antarctica studies.
In 2005, scientists working in Greenland were able to count 42,000 layers back in time, providing a good basis for dates when combined with the other data that they recorded for each layer (and sub-layer to a very high resolution) such as chemical impurities, insoluble dust, and electrolytical conductivity.
Bottom line
Only in Greenland and Japan have scientists found more than 40,000 layers; they found 42,000 in Greenland and 60,000 at Lake Suigetsu in Japan. Ages beyond these dates were estimated based on other methods, including the assumption of uniform rate of ice or sedimentation deposition and also radiometric dating.